RE: [SIP] draft-ietf-sip-session-timer-04: INVITE with Require and Support timer and Session-Expires

["Brett Tate" <brett@broadsoft.com>]

> > 
> > If a UAC both requires and supports an option-tag, 
> > should it include it in both headers?  
> 
> Its redundant. If it requires it, wouldn't that be because it 
> supports it?

It depends on if the requires option-tag was 
added by a proxy or a UA.  For instance, the proxy
currently can add timer to Require header in 2xx
responses as a means to indicate who is supposed 
to be sending refreshes.  However if require 
implies supported, this would also mean that the 
UAC can assume that timer is supported by 
the UAS which isn't true.

> > Isn't the "require implies supported" going to limit 
> > devices that want to require others to support an 
> > extension without them actually supporting it?
> 
> I can't think of a case where such a thing would make sense. 
> Do you have a
> specific example?

Actually the timer draft is a good example as 
reflected in the example of a proxy adding 
require timer.  The receiver would incorrectly 
think that the UA supported something that it 
didn't.

I currently can't recall any receiver only 
extensions since they all require extension 
support by sender.  The timer draft had the 
potential until the INVITE's require timer 
only meant that the receiver supports the 
draft and not necessarily that the extension 
would be used on the call-leg and refreshed 
by receiver.

> > However without an extra parameter 
> > in Session-Expires to indicate who is refreshing, 
> > I can't think of a better solution.
> > 
> > Actually the same parameter could be used to allow 
> > for smaller Session-Expires values which are not to 
> > be refreshed.
> > 
> > Session-Expires  =  
> >   ("Session-Expires" | "x") ":" 
> >               ( SIP-date | delta-seconds ) [refresher]
> > 
> > refresher = ";" "refresher" "=" "uas"|"uac"|"none"
> > 
> 
> What would be the usage of none? I don't see the value in that...??

The "none" would indicate neither party will be 
extending the session; thus please allow the 
small session-expires value.


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